Field of the Invention
The present disclosure relates to an X-ray diffractive grating and an X-ray Talbot interferometer having an X-ray diffractive grating.
Description of the Related Art
An imaging method utilizing X-ray absorption by a subject has been widely used in various medical and industrial fields. Further, an imaging method utilizing a phase shift, when an X-ray passes through a subject, is developed recently. Especially, attention is made to a feature that a subject configured of a light element can be imaged with high sensitivity. Especially, a Talbot interferometer which is an imaging method utilizing an X-ray diffractive grating is widely researched.
A Talbot interferometer generally uses a plurality of “gratings” having a minute periodic structure for applying amplitude modulation or phase modulation to an incident X-ray. A “beam splitter grating”, which is one of them, is a diffractive grating that diffracts an incident X-ray by a periodic structure, and forms a minute interference pattern at a predetermined position downstream of the beam by the Talbot effect. The interference pattern is called a self-image. A beam splitter grating is generally arranged near a subject in the upstream or downstream of the subject. Due to the presence of a subject, not only a change in the amplitude due to X-ray absorption by the subject but also a phase shift are caused in the transmitted X-ray. Further, scattering of the X-ray is also caused by minute fluctuations in the amplitude or phase distribution. Thereby, in the interference pattern, distortion or deterioration in visibility (also called contrast), reflecting the characteristics of the subject, occurs. By measuring changes in the interference pattern caused by the subject and performing various types of analysis, it is possible to acquire a larger amount of information than that of imaging only using absorption contrast as in the conventional case. It should be noted that as a larger amount of X-ray can be used by increasing the X-ray transmissivity of the grating, a phase modulation grating (also called phase grating) is frequently used as a beam splitter grating, rather than an amplitude modulation grating.
An interference pattern formed by a beam splitter grating is generally a minute pattern having a period of about several m. As such, in order to make it easy to perform pattern detection by an X-ray detector, an “analyzer grating”, which is another grating, is often used. By matching the pitch of the analyzer grating to the period of the interference pattern, moire having a long period can be generated by superimposition of the two. As such, it is possible to acquire information of the interference pattern by a detector not having a sufficiently high spatial resolution. Further, as the phase of the moire can be shifted by moving the grating in the periodic direction, pattern analysis by the phase stepping method can be made. Further, with this configuration, even in the case where the period of the moire is very long (longer than the width of the detection range of the detector, for example), imaging of the structure of the details of the subject can be made by the spatial resolution not depending on the moire period. It should be noted that as the analyzer grating, an amplitude modulation grating (absorption grating) is often used.
Further, in order to enable an X-ray source, in which the size of an X-ray emission spot is not sufficiently small, to be used, a “source grating” may be used as a third grating. As the source grating, an amplitude modulation grating (absorption grating) is used. This grating is generally arranged near the X-ray emission spot of the X-ray source, and works to virtually divide the X-ray emission spot having a certain spatial expanse into a plurality of minute X-ray emission spots by the periodic structure. Each virtual X-ray emission spot is small in such a degree that the fringe visibility of the interference pattern generated by the action of the beam splitter grating is maintained at a certain level or more, and the virtual X-ray emission spots are arranged in a period such that interference patterns formed by adjacent virtual X-ray emission spots superimpose with each other while being shifted by the integer multiple of the period. Thereby, although a plurality of interference patterns superimpose with each other when the actual size of the X-ray emission spot is large, periodic intensity distribution of high visibility can be formed. It should be noted that a Talbot interferometer using a source grating based on such a principle is generally called a Talbot-Lau interferometer.
A grating used in an interferometer is generally a one-directional grating having a grating pattern including periodic components in only one direction. Meanwhile, a two-dimensional grating having a grating pattern including periodic components in two or more directions, like a grating pattern of a square lattice shape, may be used. A Talbot interferometer using a two-dimensional grating has an advantage that magnitude of refraction and scattering of an X-ray by a subject can be measured in a plurality of directions.
As a phase modulation pattern of a phase grating using as a beam splitter grating, various patterns can be used. U.S. Pat. No. 5,812,629 discloses some phase modulation patterns suitable for forming an interference pattern of high visibility by the Talbot effect. In most cases, a simple modulation pattern configured of two levels, namely a phase advance portion and a phase delay portion, is used in practice, due to easiness in fabrication of the grating. For example, a one-dimensional grating having a structure in which phase advance portions and phase delay portions of the equal width are alternately arrayed and giving phase modulation of a height of π/2 to an X-ray of assumed photon energy is one of phase gratings which are often used, generally.
Further, “X-ray Talbot Interferometry with Capillary Plates” by A. Momose and S. Kawamoto, Japanese Journal of Applied Physics, Vol. 45, No. 1A, 314-316 (2006), describes a Talbot interferometer in which capillary plates are used for a beam splitter grating and an analyzer grating, as two-dimensional absorption gratings.
In a Talbot interferometer, the amounts relating to phase shift and scattering by a subject are measured according to changes in the interference pattern formed by the Talbot effect. As such, increasing the visibility of the detected interference pattern is an important element for performing highly reliable measurement.
Meanwhile, the Talbot effect is an effect based on a diffraction phenomenon of a wave, and the diffraction phenomenon is generally a phenomenon depending on the wavelength. Further, the phase modulation amount itself, by the phase grating, also changes according to the wavelength of the X-ray. Accordingly, the visibility of an interference pattern at a particular position downstream of the phase grating generally changes complicatedly by the wavelength of the X-ray to be used.
Further, wavelength dependency of the visibility of such an interference pattern is unique to the structure of a diffractive grating. In the present description, such wavelength dependency (may also be considered as photon energy dependency) of the visibility of an interference pattern may be called achromaticity of a diffractive grating. It should be noted that achromaticity is high indicates that wavelength dependency of the visibility of an interference pattern is low.
An X-ray available by an X-ray source such as an X-ray tube, which is generally used, has relatively wide energy spectrum. As such, one in which achromaticity of the phase grating is high can form an interference pattern of high visibility at a particular position, by using a wide range of energy components of the irradiated X-ray, and such components can be used for measurement.